1. Field of the Invention
This invention relates to a process and apparatus for measuring the gaseous content of industrial process gas streams, in which a portion of the process gas stream is extracted, conditioned for analysis, and analyzed on a gas analysis system using electrochemical gas sensors.
The invention relates more specifically to a gas analysis system for measuring total reduced sulfur (TRS) and other gases, which would meet or exceed the requirements of the U.S. Environmental Protection Agency (EPA), while greatly reducing cost and maintenance requirements of existing systems.
2. Description of Related Art
Total reduced sulfur, is defined by the EPA, to be the sum of hydrogen sulfide methyl mercaptan, dimethyl sulfide and dimethyl disulfide. SO2 is not included. Total reduced sulfur compounds are measured by removing any SO2 that coexists with the TRS gases using an SO2 scrubber, passing the remaining gases through a thermal oxidizer to convert the remaining TRS compounds to SO2, and then analyzing the SO2 using a fluorescence type analyzer.
Existing TRS analyzer systems are very expensive to manufacture, install and maintain. A typical TRS analyzer system contains the following components:
1. A sampling probe/conditioning system that is installed on the boiler or kiln stack;
2. A tubing umbilical to transport the gases to the analyzer location;
3. An air-conditioned shelter for the TRS analysis system;
4. A fluorescence type SO2 analyzer;
5. An SO2 scrubber;
6. A thermal oxidizer;
7. Stainless steel sample pump; and
8. A regenerative air cleanup system.
Existing TRS analyzer systems may cost as much as $50,000.00 and the installation cost can easily double or triple that amount for analyzer shelters and umbilical tubing installation. Maintenance on these systems is complicated because the system may be spread out over 500 feet or more between the sample probe location and the analyzer location. These systems are permanently installed and must be serviced by field technicians. Due to analyzer response time requirements of the EPA and the flow required by the fluorescence type SO2 analyzer, relatively large flows (3000 cc/min) must be pulled from the stack and 400 to 600 cc/min must be supplied to the analyzer. These gas flow rates set the size of the SO2 scrubber, the thermal oxidizer and other system components. These systems have been highly developed over the years and little more can be done to reduce the cost of this type of system.
Attempts to lower the cost of the system by replacing the fluorescence type SO2 analyzer with electrochemical sensors have had limited success. To transport the TRS gases over long distances without sample loss requires the use of a dilution type probe that provides a very dilute (50:1 dilution) gas to the analyzer. Electrochemical SO2 sensors generally have trouble measuring these low concentrations. Another problem is that the electrochemical sensors are damaged by the extremely dry gas supplied by the dilution sampling system. Electrochemical sensors may also be damaged by too much moisture; too little and the sensor dries out, too much and condensation may form inside the sensor causing the sensor to fail. The extremely dry gas also dries out the SO2 scrubber media, which must be back flushed with humidified air every fifteen minutes. The SO2 scrubber distilled water reservoir must be filled at least once each week.
It is therefore an object of the invention to provide a system for monitoring TRS of relatively lesser size and cost than prior art system.
It is a further object of the invention to provide a system for monitoring TRS utilizing a low-cost electrochemical sensor for SO2.
To achieve these and other objects the invention provides a TRS analyzer and gas conditioning system that is advantageously contained in one enclosure at the stack location. The system includes a sampling probe for withdrawing stack gas, a filter for removing particulate matter from withdrawn stack gas, a heat exchanger for precisely regulating the temperature of the filtered gas, a scrubbing column for removing SO2 from the temperature regulated gas, splitting means for splitting the scrubbed gas into two portions, an oxidizer for oxidizing a first portion of the scrubbed gas to covert total reduced sulfur compounds to SO2, a first electrochemical sensor for determining SO2 in the converted gas, a second electrochemical sensor for determining oxygen in a second gas portion, and means for regulating the temperature of the first and second sensors to a value substantially the same and which is at least equal to the temperature of the heat exchanger. This system does not require a tubing umbilical, air-conditioned shelters, sample pumps or regenerative air dryers. The entire system can be contained in a 24xe2x80x3Wxc3x9730xe2x80x3Wxc3x978xe2x80x3D enclosure and weigh 65 pounds. This TRS analyzer can be manufactured at a much lower cost and requires less maintenance.
The TRS system of the invention was designed to solve the problems associated with using electrochemical sensors to measure TRS and O2. This system uses low flow rates to allow for long sample filter life and to allow all gas conditioning components to be miniaturized to reduce cost. Installing the analyzers at the stack location eliminates the need for a tubing umbilical. Locating the analyzers close to the source also allows the analyzers to respond quickly even with low sample flow rates.
The invention also provides a method for analyzing gas flowing through a stack for total reduced sulfur comprising the steps of withdrawing a portion of the gas flowing through the stack, filtering the withdrawn gas, regulating the temperature of the filtered gas to a predetermined value, scrubbing SO2 from the temperature regulated gas in a column at substantially the same temperature as the regulated gas, splitting the scrubbed gas into first and second portions, oxidizing a first a first portion of the scrubbed gas to convert total reduced sulfur compounds to SO2, analyzing the converted gas for SO2 with an electrochemical sensor maintained at a temperature at least equal to the regulated gas, and analyzing the second portion of the scrubbed gas for oxygen utilizing a second electrochemical sensor maintained at a temperature at least equal to the regulated gas and which is substantially the same as the first sensor.
By measuring the stack gas directly, without dilution, and installing the SO2 scrubber into the same temperature controlled zone as the heat exchanger, the need for water addition to the scrubber has also been eliminated. By placing the electrochemical sensors into a separate temperature controlled zone and controlling the temperature difference between the gas heat exchanger and electrochemical sensors, precise control over the humidity level of the gas can be maintained. This allows the electrochemical sensors to operate long term with minimum drift.